1. Field of the Invention
This invention relates to methods and apparatus for automated job scheduling in a data storage and/or retrieval system.
2. Description of the Prior Art
Centralised computer data storage and/or retrieval systems have been proposed in which the data storage and/or retrieval system is shared between many "client" computers--i.e. computers individually requiring to store or retrieve a quantity of data.
An example of such a system is a video archiving system for professional use--e.g. by a television broadcasting station. Here, a central archive of very many hours of video material might be stored on digital video tapes. Since the amount of archived material to be stored is likely to be very much more than can be fitted onto one tape, it has been proposed that an automated tape handling mechanism such as a robotic tape changer could be used. A robotic tape changer of this type is typically able to unload a tape cassette from a tape recording/replay machine (e.g. a video tape recorder or VTR), to return the tape cassette to the correct place in a tape cassette shelving or racking system, to pick up the next tape cassette from the racking system and to load it into the VTR.
However, these operations all take time. There is a certain amount of time associated with the actions of the robotic changing mechanism, and also a significant (and often much greater) amount of time needed to rewind and unload a current tape cassette, and then to load and cue up a next tape cassette. These time penalties would be present simply because a storage medium based on multiple storage volumes (tape cassettes) is in use; the robotic arrangement simply adds to the already-existing problem.
So, there is a need to reduce, wherever possible, the time taking in accessing data held on a multi-volume storage medium.